1. Field of the Invention
The invention relates to a motor control device for driving and controlling a motor. More specifically, the invention relates to a technology for detecting a shortage of a supply voltage to the motor that causes a step-out or for preventing an occurrence of the step-out.
2. Description of Related Art
FIG. 1 shows a block diagram of a conventional motor drive system. The motor drive system of FIG. 1 includes a three-phase permanent magnet synchronous motor 901, a PWM inverter 902, and a motor control device 903. In the motor drive system of FIG. 1, a d-axis current id and a q-axis current iq are computed by a dq conversion of a U-phase current iU and a V-phase current iV that are detected by a current sensor. Then, current feedback control is performed such that the d-axis current id and the q-axis current iq follow a d-axis current command value id* and a q-axis current command value iq*, and three-phase voltage command values (vu*, vv*, and vw*) are generated from a d-axis voltage command value vd* and a q-axis voltage command value vq* obtained from this feedback control. The q-axis current command value iq* is set based on a rotation speed command value ω* for specifying the rotation speed of the motor 901. The PWM inverter 902 converts a DC output voltage from a DC power source 904 to a three-phase AC voltage according to the three-phase voltage command values, and vector-controls the motor 901 by supplying the three-phase AC voltage to the motor 901.
The input voltage to the inverter 902, which is an output voltage of the DC power source 904 may fluctuate in actual use. The decrease of the input voltage acts in a direction that the voltage (the voltage the motor 901 requires) to be supplied to the motor 901 from the inverter 902 for driving the motor 901 runs short. Such a shortage of the supply voltage causes a step-out of the motor 901.
The input voltage to the inverter 902 itself usually is monitored by a voltage detector (not shown). However, it is difficult to determine shortages of the supply voltage to the motor 901 from the input voltage information. This is because the voltage required for driving the motor 901 depends on operating conditions that include various elements such as the rotation speed and the torque of the motor 901. Therefore, it is difficult to determine a possibility of the step-out of the motor 901 from the input voltage information.
The increase in the load torque also acts in a direction for the shortage of the supply voltage to occur. This is because when the load torque is increased, a larger motor drive voltage becomes necessary to generate the torque corresponding to the load torque.
Weak magnetic flux control with a negative d-axis current generally is used to prevent a step-out derived from the supply voltage shortage. By performing the weak magnetic flux control, an induced voltage generated within the motor 901 is decreased and the shortage of the supply voltage is resolved or suppressed. However, when the decrease of the input voltage to the inverter 902 or the increase of the load torque is precipitous, such resolution or suppression of the supply voltage shortage by the weak magnetic flux control may be too late due to the control delay and it may result in the step-out.
In recent years, with the reduction in size and weight of motors, the motors are often driven near the threshold within a rated operation range. Although the supply voltage tends to run short at the high-speed rotation range, the rated operation range is expanded to the high-speed rotation range by performing the weak magnetic flux control. In order to achieve the reduction in size and weight as well as the high-speed rotation at the same time, the motor is driven near the voltage limitation especially at the time of performing the weak magnetic flux control, and relatively small decrease in the input voltage or relatively small increase in the load torque causes the supply voltage shortage that causes the step-out.
Several methods have been proposed for detecting an occurrence of this kind of step-out. For example, one conventional method performs detection of the step-out based on a d-axis current detected in a state that the d-axis current command value is made zero (see for example Japanese Patent Laid-Open No. 2001-25282). In another conventional method, the step-out is detected based on a certain physical quantity (such as a correction amount with respect to the rotation speed command) that has a distinctive value at the time of an occurrence of the step-out (see for example Japanese Patent Laid-Open No. 2003-79200). In another conventional method, the d-axis current command value (d-axis current target value) is varied periodically and detection of the step-out is performed based on a q-axis current response at that time (for example, see Japanese Patent Laid-Open No. 2005-151635).
However, the above conventional methods detect an occurrence of the step-out after the step-out has occurred, and cannot detect the shortage of the supply voltage that causes the step-out. In other words, these conventional methods cannot prevent an occurrence of the step-out.
The invention aims to provide a motor control device that contributes to the detection of the supply voltage shortage or prevention of an occurrence of the step-out, and a motor drive system and a compressor utilizing such a motor control device.